1. Field of the Invention
The present invention generally relates to a method for improving image fidelity on a semiconductor wafer and more particularly, to a method of controlling electron beam intensity of an electron beam projection lithography system in order to provide a uniform feature size throughout a subfield of a resist pattern.
2. Background Description
Semiconductor manufacturers typically use lithographic processes in a highly specialized printing process to put detailed patterns onto silicon wafers. In general, a layer of photosensitive material called "resist" is deposited onto a silicon wafer and an image containing the desired pattern of energy or charged particles is projected onto the silicon wafer. After development, the resist forms a stenciled pattern (e.g., image) across the wafer surface that matches the desired pattern of a circuit.
More specifically, in electron beam lithography, finely focused electron beams are emitted from a cathode surface and through a series of optical lenses and a patterned reticle in order to provide a fine line pattern or image on the resist surface. In common practice, a 4x reticle is electron optically imaged and then demagnified onto the writing surface (e.g., resist). In common practice, the 4x reticle includes a square subfield of 1 mm width which results in a 0.25 mm wide subfield image on the resist. However, 1x, 2x, 3x, etc. reticles may also be used in lithographic systems.
FIG. 1 shows an electron-beam apparatus of common design. Specifically, in the apparatus of FIG. 1, electrons are emitted from a hot cathode, for example, approximately 1700.degree. K, in a highly uniform intensity distribution. The electrons are then emitted from a charged particle emitting device 10, for example, a crystal, forming an electron beam 12 having a uniform intensity distribution. The electron beam 12 passes through a series of optics (e.g., auxiliary lens 14, condenser 16 and an illuminator doublet 20) prior to being optically imaged on a reticle 22 and then demagnified onto the target 28 (e.g., resist). Prior to the electron beam being demagnified onto the target 28, it first passes through a series of optics (e.g., projection doublet 24 and contrast aperture 26) to ensure proper focus of the electron beam 12 onto the target 28. The dashed lines 30 of FIG. 1 represent lateral areas being imaged and the solid vertical line 32 of FIG. 1 represents an ideal path for the electron beam 10.
A typical problem presented with the use of electron beam lithography is degradation of image fidelity due to naturally occurring electron optical aberrations occurring within the subfield. Aberrations are typically defined as the variation of pixel size and usually vary symmetrically in the radial direction about the subfield center.
Aberrations usually arise from two sources: (i) geometric field aberrations of the focusing and deflection system, and (ii) space charge interaction within the beam. Geometric field aberrations are independent of the pattern being printed on the wafer surface and are predictable by computation in the limit where the system is well aligned, and a stigmatic image is obtained on the central axis. However, aberrations caused by space charge interaction depend on the pattern being printed on the wafer surface insofar as this determines the local current density within the electron beam path which, in turn, governs the space charge interaction. Aberrations caused by space charge interaction are also predictable by computation. The resultant field aberration arising from both (i) geometric field aberrations and (ii) space charge interaction are measurable for a given set of operating conditions.
Aberrations cause image blurring which effects the intensity distribution or exposure dose (intensity.times.time) in the image which, in turn, also affects the printed line width on the resist. Blurring is generally manifested as a gray level phenomenon and occurs when the image is not completely developed on the resist.
As the blurring increases, the intensity (or exposure dose) is spread over a wider area and the intensity of the center of the feature decreases. This leads to a narrowing of the printed image because of the dose threshold for full development and the high contrast nature of the preferred resists. If the blurring is sufficiently pronounced, the image may not print at all as the central intensity becomes insufficient to expose the resist.
It is common practice to compensate for image size variations in the reticle by choosing feature sizes in the reticle to compensate for these variations. This necessitates incorporating the image size information at the reticle fabrication step which significantly increases the complexity of the overall process. Furthermore, the reticle can not be changed once it is fabricated, and if the reticle needs to be changed, the entire process must be stopped. This greatly increases manufacturing costs and time.